For the cheese spaetzle, cook the spaetzle according to package directions. Mix the cooked spaetzle with the grated cheese and place in a portion baking dish.
Bake the spaetzle in a preheated oven at 180 °C for about 10-15 minutes. In the meantime, peel the onion and cut it into fine rings. Heat the butter in a pan, sauté the onion rings in it until translucent and lightly brown.
Spread the browned onion rings on the spaetzle and serve immediately.